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Marion Nestle, a nutritionist, has been calling for sweeping change in the food industry.

AS tens of thousands of people recently strolled among booths of the nation’s largest organic and natural foods show here, munching on fair-trade chocolate and sipping organic wine, a few dozen pioneers of the industry sneaked off to an out-of-the-way conference room.

Although unit sales of organic food have leveled off and even declined lately, versus a year earlier, the mood among those crowded into the conference room was upbeat as they awaited a private screening of a documentary called “Food Inc.”  a withering critique of agribusiness and industrially produced food.

They also gathered to relish their changing political fortunes, courtesy of the Obama administration.

“This has never been just about business,” said Gary Hirshberg, chief executive of Stonyfield Farm, the maker of organic yogurt. “We are here to change the world. We dreamt for decades of having this moment.”

After being largely ignored for years by Washington, advocates of organic and locally grown food have found a receptive ear in the White House, which has vowed to encourage a more nutritious and sustainable food supply.

The most vocal booster so far has been the first lady, Michelle Obama, who has emphasized the need for fresh, unprocessed, locally grown food and, last week, started work on a White House vegetable garden. More surprising, perhaps, are the pronouncements out of the Department of Agriculture, an agency with long and close ties to agribusiness.

In mid-February, Tom Vilsack, the new secretary of agriculture, took a jackhammer to a patch of pavement outside his headquarters to create his own organic “people’s garden.” Two weeks later, the Obama administration named Kathleen Merrigan, an assistant professor at Tufts University and a longtime champion of sustainable agriculture and healthy food, as Mr. Vilsack’s top deputy.

Mr. Hirshberg and other sustainable-food activists are hoping that such actions are precursors to major changes in the way the federal government oversees the nation’s food supply and farms, changes that could significantly bolster demand for fresh, local and organic products. Already, they have offered plenty of ambitious ideas.

For instance, the celebrity chef Alice Waters recommends that the federal government triple its budget for school lunches to provide youngsters with healthier food. And the author Michael Pollan has called on President Obama to pursue a “reform of the entire food system” by focusing on a Pollan priority: diversified, regional food networks.

Still, some activists worry that their dreams of a less-processed American diet may soon collide with the realities of Washington and the financial gloom over much of the country. Even the Bush administration, reviled by many food activists, came to Washington intent on reforming farm subsidies, only to be slapped down by Congress.

Mr. Pollan, who contributes to The New York Times Magazine, likens sustainable-food activists to the environmental movement in the 1970s. Though encouraged by the Obama administration’s positions, he worries that food activists may lack political savvy.

“The movement is not ready for prime time,” he says. “It’s not like we have an infrastructure with legislation ready to go.”

Even so, many activists say they are packing their bags and heading to Washington. They are bringing along a copy of “Food Inc.,” which includes attacks on the corn lobby and Monsanto, and intend to provide a private screening for Mr. Vilsack and Ms. Merrigan.

“We are so used to being outside the door,” says Walter Robb, co-president and chief operating officer of Whole Foods Market, the grocery chain that played a crucial role in making organic and natural food more mainstream. “We are in the door now.”

AT the heart of the sustainable-food movement is a belief that America has become efficient at producing cheap, abundant food that profits corporations and agribusiness, but is unhealthy and bad for the environment.

The federal government is culpable, the activists say, because it pays farmers billions in subsidies each year for growing grains and soybeans. A result is an abundance of corn and soybeans that provide cheap feed for livestock and inexpensive food ingredients like high-fructose corn syrup.

They argue that farm policy  and federal dollars  should instead encourage farmers to grow more diverse crops, reward conservation practices and promote local food networks that rely less on fossil fuels for such things as fertilizer and transportation.

Last year, mandatory spending on farm subsidies was $7.5 billion, compared with $15 million for programs for organic and local foods, according to the House Appropriations Committee.

But advocates of conventional agriculture argue that organic farming simply can’t provide enough food because the yields tend to be lower than those for crops grown with chemical fertilizer.

“We think there’s a place for organic, but don’t think we can feed ourselves and the world with organic,” says Rick Tolman, chief executive of the National Corn Growers Association. “It’s not as productive, more labor-intensive and tends to be more expensive.”

The ideas are hardly new. The farmland philosopher and author Wendell Berry has been making many of the same points for decades. What is new is that the sustainable-food movement has gained both commercial heft, with the rapid success of organic and natural foods in the last decade, and celebrity cachet, with a growing cast of chefs, authors and even celebrities like Oprah Winfrey and Gwyneth Paltrow who champion the cause.

It has also been aided by more awareness of the obesity epidemic, particularly among children, and by concerns about food safety amid seemingly continual outbreaks of tainted supplies.

While their arguments haven’t gained much traction in Washington, sustainable-food activists and entrepreneurs have convinced more Americans to watch what they eat.

They have encouraged the growth of farmers’ markets and created such a demand for organic, natural and local products that they are now sold at many major grocers, including Wal-Mart.

“Increasingly, companies are looking to reduce the amount of additives,” says Ted Smyth, who retired earlier this year as senior vice president at H. J. Heinz, the food giant. “Consumers are looking for more authentic foods. This trend absolutely has percolated through into mainstream foods.”

While the idea of sustainable food is creeping into the mainstream, the epicenter of the movement remains the liberal stronghold of Berkeley, Calif.